Film cameras of the type typically used in professional cinematographic production provide the camera operator with an optical image corresponding to the image exposed on film. With the advent of compact video components, professional motion picture cameras have been modified to employ video pickups built into the film camera in order to allow other people to view the image seen by the camera operator through the viewfinder eyepiece. Such video pickups are also employed with remotely controlled film cameras. In this latter application, the focus of the film camera lens is controlled entirely with the aid of the video image that is presented to the camera operator.
Prior art video pickups for film cameras use the image that is focused on the ground glass screen of the camera's optical viewfinder. This image is then focused on the face of a vidicon tube or a solid-state detector array by means of a system of lenses. The depth of field associated with such lenses can introduce focusing errors. While the camera operator is attempting to focus the image which appears on the surface of the ground glass focusing screen, prior art video viewfinding systems may cause the operator to focus on an aerial image that is within the depth of field of the lens system and that is immediately ahead of or behind the ground glass surface. This problem increases as ambient light levels fall and the video system looses the ability to present good contrast and acceptable image resolution on the video monitor.
Aggravating this focusing problem is the tendency of the camera operator to replace the ground glass screen with a more translucent glass in order to pass more light to the video camera. This will diminish the intensity of the image on the surface on the ground glass in comparison with a nearby aerial image. In extremely low light level situations, the operator may remove the ground glass entirely. In either case, the likelihood of a focusing error is greatly increased.
In addition to the focusing problems discussed above, prior art video viewfinding systems that view an image on the ground glass focusing screen are limited in resolution and contrast due to the granular surface of the ground glass.